The invention relates to a process for the preparation of salt melts and mixtures thereof by means of an extruder in which the starting materials are melted and brought to reaction, and the reaction products are subsequently passed through a tower or column containing alkali metal salt.
Extruders are traditionally employed for melting and mixing polymeric materials, such as plastics. The extruders can also be used as reactors for polymerizations. Anionic polymerization in an extruder is described, for example, for caprolactam to nylon 6 (B. Vanuskirk, M. K. Akkapeddi, Polym. Prepr. Vol. 29, 557 (1988)). The advantage of using an extruder rather than a stirred-tank reactor for polymerization reactions is that it is easier to process high-viscosity substances. The requirement for thorough mixing of all substances participating in the reaction and good heat transfer can also be achieved optimally by using an extruder (G. Menges et al., New Polym. Mater., Proc. Int. Semin., 129-148 (1987)). In EP 813 904, pharmaceutical active ingredients are prepared by means of extruders. Here, pharmaceutical active ingredients carrying acid groups are reacted with a base in the melt.
Melts of salts, such as, for example, NaAlCl4, have various areas of application. Salt melts can be employed as storage medium in heat stores, as heat-transfer agents, for example in heating baths, for covering and purifying molten metals, for electro-coating of high-melting materials or as melt electro-lyte in primary batteries, as described in GB 2,046,506. A further possible application of these salts is in re-chargeable sodium batteries. The salts are employed in batteries which have operating temperatures of between 130xc2x0 C. and 200xc2x0 C. (K. M. Abraham, D. M. Pasquariello, J. Electrochem. Soc., Vol. 137. 1189-1190 (1990)).
DE 3419279 describes an electrochemical cell in which the cathode matrix is impregnated with a sodium/aluminium halide salt melt electrolyte.
A relatively new area of application is the xe2x80x9cZEBRA batteryxe2x80x9d. This high-temperature cell consists of an electrode of liquid sodium, a beta-aluminium electrolyte and an electrode of transition-metal chloride in an NaAlCl4 melt (B. Cleaver, V.S. Sharivker, J. Electrochem. Soc., Vol. 142, 3409-3413 (1995)).
DE 3718920 describes the preparation of salt melts by adding a pure metal and an alkali metal halide to the melt. The reaction cell is operated above the melting point of the salt melt. In the working example, the alkali metal halide is NaCl, the molten alkali metal is sodium, and the separator is beta-aluminium oxide. Owing to the use of pure sodium, special safety precautions, such as working under a protective-gas atmosphere, must be taken. The reactions must take place in separate cells, since poisoning of the separator by the by-product AlHal3 formed must be prevented.
For the preparation of the alkali metal halogen aluminates, the reaction of corresponding aluminium halides and alkali metal halides in a sealed tube is described (Friedmann, Taube, J. Am. Chem. Soc., 72, 2236-2243 (1950)). In this process, an increase in pressure to 6-7 atmospheres is observed, which results in problems (FR 2168912). The apparatuses must be fitted with the appropriate safety precautions.
All the processes disclosed hitherto for the preparation of salt melts operate batchwise. A batch procedure has some severe disadvantages compared with a continuous preparation process. During a batch change, the apparatus must be opened. The product can then be contaminated by the oxygen from the ambient air, water and dust. The batch change results in downtime of the plants and thus in a reduced space-time yield. An effective discontinuous process requires large apparatuses. The start-up process requires correspondingly more energy and time. It has been found that, in particular during start-up of the plants, impurities can be introduced into the process. FR 2168912 describes a complex purification process for alkali metal halogen aluminates. The 2-step purification process consists of oxygen treatment for degrading the organic impurities and aluminium treatment for precipitating iron and heavy metals. The aluminium treatment must be carried out under a nitrogen or argon atmosphere.
The object of the invention is to provide a continuous process for the preparation of pure salt melts which excludes the disadvantageous effects of the environment, minimizes the energy requirement and facilitates an optimum space-time yield. A further object is to make large amounts of salt melts available in the shortest possible time.
The object according to the invention is achieved by a process for the preparation of salt melts, and mixtures thereof, of the general formula
MDHalxe2x80x83xe2x80x83(I)
in which
M is Li, Na, K, Rb or Cs,
D is Al, Ga, In or Tl, and
Hal is F, Cl, Br or I,
characterized in that the starting materials, a metal halide and an alkali metal halide, are melted and brought to reaction continuously or, if desired, discontinuously in a heatable extruder with forced conveying, and the reaction products are subsequently passed through a tower or column containing alkali metal salt.
The process products are suitable for use as melt electrolyte in electrochemical cells, as storage medium in heat stores, as heat-transfer agent, for example in heating baths, for covering and purifying molten metals, for electrocoating of high-melting materials or as melt electrolyte in rechargeable sodium batteries and primary batteries.
Surprisingly, it has been found that the starting materials can be processed by means of an extruder. It is possible to prevent the feared contamination of the product by abrasion by means of a suitable choice of material and residence time.
It has been found that forced conveying in the extruder by pumps can be omitted in this process, which considerably reduces the susceptibility of the process to faults.
Any extruder which appears suitable to the person skilled in the art can be used for the process. Particularly suitable extruders are single-screw extruders, multiscrew extruders with co-rotating and counterrotating screws, vented extruders, planetary-gear extruders, ram extruders and disc extruders. For the processing of salts in extruders, the hardness of the substances employed and their chemical properties represent particular problems which have hitherto stood in the way of implementation. In general, the extruder is made of steel. This material would be badly damaged by corrosion and abrasion during processing of salts.
It has been found that corrosion can be greatly reduced in extruders whose essential components are made of nickel alloys. It has furthermore been found that the metal parts of the extruder which come into contact with the salts or their melts can be protected against corrosive and abrasive damage by surface coatings with materials known to the person skilled in the art, such as PTFE/PFA, enamel or ceramic materials. In order to reduce abrasion, an additional bearing can be installed at the head of the screw.
Surprisingly, it has been found that the installation of screw elements with a reversed flow direction allows a completely molten and homogeneous product to be obtained in spite of a very short average residence time of the material in the extruder of a few seconds.
The reaction in the extruder can be carried out in the presence of atmospheric oxygen or, if desired, under a protective-gas atmosphere (for example nitrogen, CO2 or noble gases) at a reduced pressure, atmospheric pressure or even at superatmospheric pressure at temperatures of from 50xc2x0 C. to 800xc2x0 C. (at atmospheric pressure). When working under superatmospheric pressure or reduced pressure, the melting points of the salts shift correspondingly and the heating stages of the extruder are modified correspondingly.
Processing should be carried out below the sublimation temperature of the starting materials. The reaction is preferably carried out at elevated temperatures, since the solubility of the salts is significantly better under such conditions.
During processing of the salts in the extruder, a specific choice of temperature in the heating stages allows an optimum temperature programme to be set during the process.
In order to carry out the process, the aluminium halide employed is a fluoride, chloride, bromide or iodide, or mixtures thereof. Suitable alkali metal salts are lithium, sodium, potassium, rubidium or caesium fluoride, chloride, bromide or iodide, or mixtures thereof.